1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus adaptive to a plurality of different display modes and having a high pixel density, synchronous video output capability and a method of signal processing. Particularly, the image pickup apparatus of the present invention is advantageously applicable to a digital camera, image input device or similar imaging equipment having a great number of pixels, e.g., several hundred thousands to several millions of pixels.
2. Description of the Background Art
Digital cameras available on the market today include an electronic camera using as many as, e.g., several hundred thousands to several millions of photosensitive cells, or pixels, constituting the basic devices of an image pickup section. With such a great number of photosensitive cells, the electronic camera is expected to realize image quality comparable with one achievable with a traditional silver-halide sensitive type of camera. It is a common practice with the electronic camera to write all or part of pixel data constituting a picture in a frame memory. The pixel data may be read out of the frame memory under the control of a system controller and displayed on a display in the form of a picture. In the event of reading of the pixel data, the system controller controls the frame memory on the basis of the vertical rate and horizontal rate of an NTSC (National Television System Committee), PAL (Phase Alternation by Line) or similar current broadcast system adaptive to a display. Consequently, an image pickup section or input section for picking up a scene and a display section or output section for displaying data output from the pickup section are not synchronous in timing to each other.
To synchronize the image pickup section and display section with respect to pixel data, it is necessary to increase the rate at which the pixel data are read out of the image pickup section or to skip, or reduce, the pixel data output from the above section at preselected intervals.
Assume that a signal representative of a scene picked up is read out of the image pickup apparatus at a high speed for implementing, e.g., an asynchronous movie. Then, it is difficult to make the most of the expected functions of various devices, e.g., noise reduction based on the correlation double sampling of the signal, signal amplification, and valid bits of analog-to-digital conversion. As a result, the signal output from the image pickup apparatus is lowered in SN (Signal-to-Noise) ratio. Further, this kind of scheme obstructs matching to be effected during, e.g., signal processing by a digital encoder later and aggravates power consumption due to the high-speed processing.
On the other hand, the pixel skipping or reduction scheme reduces the number of pixels only in the vertical direction and therefore sacrifices the vertical resolution of a picture. For example, assume that the number of pixels of the image pickup section is four times as great as a VGA (Video Graphics Array) size, and that data are read out of the pixels in accordance with a clock having a frequency of 12 MHz and synchronously displayed on a display of VGA size. Then, it is necessary to reduce the number of pixels to be read to one-fourth. In practice, however, the pixels are reduced to one-eight in the vertical direction field by field in the NTSC or the PAL system so as to implement one-fourth reduction relatively easily. In this case, the pixels are not reduced in the horizontal direction because horizontal reduction is difficult to perform due to the arrangement of the photosensitive devices of the image pickup section. As a result, one horizontal period of the image pickup corresponds t two horizontal periods for the display. The picture reduce only in the vertical direction is ill balanced, compared to a picture reduced by half in both of the horizontal and vertical directions.